The present invention relates generally to a pressure sensor for detecting the average pressure of a fluid such as air vapor or liquid. More particularly, the present invention relates to a pressure sensor for detecting variation of pressure in use with a pressure sensitive elastic or expansible member.
FIG. 1 shows a conventional pressure sensor being applicable for detecting pulsating pressure such as an exhaust gas pressure of a reciprocating internal combustion engine. The pressure sensor 10 generally comprises a sensor casing 12 having therein a pressure chamber 14 defined by the internal periphery of the sensor casing 12 and a diaphragm 16 of flexible or elastic material. The pressure chamber 14 communicates with a pressure introducing passage 18 through which pressurized fluid such as air, vapor or liquid of which the pressure to be detected is introduced into the pressure chamber 14. An electrically conductive movable contact 20 is movably received within another chamber 22 which is also defined by the internal periphery of the sensor casing 12 and the diaphragm 16 at the opposite side of the pressure chamber 14 with respect to the diaphragm 16. The movable contact 20 is connected to an electrode 24 adjacent the outer periphery thereof. The electrode 24 is connected with a smoothing circuit 26 for smoothing the sensor output and is inserted into the chamber 22 through an opening 28 formed in the casing. The movable contact 20 is formed by an elastic and electrically conductive material and is provided, at the central portion thereof with a movable terminal 30. Opposing the terminal 30, the other electrode 32 is inserted to the chamber 22 through an opening 24 formed in the casing 12.
A set spring 36 for applying to the diaphragm 16 an initial set pressure P.sub.0 is disposed within the pressure chamber 14. When the positive pressure introduced through the passage 18 exceeds the set pressure P.sub.0, the diaphragm 16 is pushed toward the electrode 32 by the introduced pressure and thereby the terminal 30 contacts the electrode to complete closed circuit. A return spring 35 for returning the movable contact 20 to neutral position is disposed within the chamber 22. A stopper 37 is also provided within the chamber 22 to limit movement of the movable contact in the direction approaching the diaphragm 16. In other words, the electrode 32 and the terminal 30 form an ON/OFF switch turning ON or OFF in response to the pulsating pressure introduced within the pressure chamber 14.
When the pulsating pressure of the air, vapor or liquid is introduced into the pressure chamber 14 and the pressure is applied to the diaphragm 16, output of the switching means consisting of the terminal 30 and the electrode 32 is a pulse signal. FIG. 2 shows variation of the electric voltage at a point 38. The pulse output is inputted to the smoothing circuit 26 and is smoothed in order to obtain an average output E.sub.m. The average output E.sub.m of the smoothing circuit 26 is calculated arithmetically to obtain an average pressure P.sub.m of the pulsatile pressure.
In such a conventional pressure sensor, the set pressure applied to the diaphragm 16 by the set spring 36 is determined so that it is between the maximum and minimum pressure of the pulsating pressure to be detected. Therefore, if the pressure applied to the diaphragm 16 is relatively high with respect to the set pressure P.sub.0, the period for which the diaphragm 16 is urged toward the electrode 32 is relatively long. Namely, the period for which the diaphragm 16 is urged toward the electrode and thereby the terminal 30 contacts to the electrode 32 is proportional to the pressure applied to the diaphragm. In this conventional sensor, the pressure value is converted to this time period and by measuring the period corresponding to the applied pressure, the value of pressure is determined. Therefore, if a constant pressure is introduced into the pressure chamber, the diaphragm will be stationary in the neutral position if the pressure is less than the set pressure P.sub.0 and stationary in the urged position if the pressure exceeds the set pressure. Thus, converting the pressure to the time period and measuring the period is practically impossible in this case. Further, if a pressure with a small pulse height is introduced into the pressure chamber, it is also impossible to detect a pressure exceeding the reference pressure P.sub.0, and therefore it is impossible to determine the pressure.